When cells are exposed to radiations or chemicals that damage DNA, several mechanisms are available to mend the lesions induced. However, some DNA damage can remain unrepaired or be misrepaired, and can have deleterious consequences, including mutagenesis, carcinogenesis, teratogenesis or cell death. DNA damaging agents are present in the environment, and are also used routinely for cancer treatment and disease diagnosis. Therefore, it is important to understand how cells process DNA damage. The candidate primarily uses the fission yeast Schizosaccharomyces pombe as a model system to do so. This eukaryote affords many advantages over working with mammalian cells in the lab, including ease of cell growth, the ability to study haploids, diploids and tetrads, and the availability of advanced molecular techniques useful for genome manipulation. Yet, in addition to these benefits, t his organism is remarkably similar to mammals in terms of genome organization, expression of genetic material, and conservation of DNA repair and other metabolic functions. S. pombe rad9 and rad23 play key roles in promoting radioresistance since cells containing mutations in these for repair of damage before entry into mitosis, an event that could be lethal if attempted when DNA lesions are present. Thus, rad9 serves as a link between DNA damage and cell cycle regulation. The candidate's research program includes a multi-faceted approach to study the functions of rad9 and rad23. Studies include; 1) cloning and structural analyses of the genes and characterization of the encoded proteins from S. pombe, as well as the identification of functional homologues from other organisms; 2) identification and characterization of proteins that interact with rad9 or rad23 to identify processes within which they participate; 3) analyses of the biochemistry of DNA repair using a S. pombe cell-free system made from wild type and mutant cell extracts; and 4) analyses of mutagenesis -i.e., error-prone DNA repair, in wild-type and mutant cells using a shuttle vector system. Taken together, the goal of these studies is to define the molecular mechanisms used by cells to repair DNA damage and ultimately promote radioresistance.